The present invention relates generally to the field of geophysical exploration. Specifically, a novel method and system of obtaining, interpreting, processing, and displaying seismic data having improved quality as well as new information concerning the earth's subterranean formations are disclosed. Additionally, an improved shear wave seismic exploration technique is disclosed in which a shear wave seismic survey line of profile can be laid without prior knowledge of or regard for the geological character of the earth's subterranean formations.
Historically, shear wave seismic exploration techniques have employed shear wave seismic sources and shear wave seismic receivers in a seismic survey to gather seismic data. Such a seismic survey has been either linear or areal in its extent. The seismic energy imparted by the shear wave seismic source is detected by the shear wave seismic receivers after interacting with the earth's subterranean formations. Such seismic surveys, however, have been limited to utilizing a shear wave seismic source having a single line of action or polarization, oriented with respect to the seismic survey line of profile, to preferentially generate seismic waves of known orientation, e.g., horizontal shear (Sh) waves or vertical shear (Sv) waves. The shear wave seismic receivers utilized in conjunction with a given shear wave seismic source have similarly been limited to a single line of action or polarization, oriented with respect to the seismic survey line of profile, to preferentially receive a single component of the seismic wave, e.g., (Sh) wave or (Sv) wave. As used herein, the term "line of action" generally comprehends a defined vector displacement, such as the particle motion of the seismic wave. In present shear wave seismic surveys, the lines of action of the seismic source and the seismic receivers have the same orientation relative to the line of profile and are said to be "matched".
As long as seismic surveys were limited to seismic sources and seismic receivers having a compressional (P) wave lines of action, satisfactory results were generally obtained irrespective of the orientation of the seismic survey line of profile with respect to the underlying geological character of the subterranean formations. However, when the seismic sources and seismic receivers are of the shear wave type, i.e., either horizontal shear (Sh) wave or vertical shear (Sv) wave, the orientation of the seismic survey line of profile and/or the line of action of the shear wave seismic source with respect to the geological character of the subterranean formations can determine whether or not meaningful seismic data is obtained.
As understood by those skilled in the art, compressional (P) waves are longitudinal waves where the particle motion is in the direction of propagation. Shear waves are transverse waves where the particle motion is in a transverse plane perpendicular to the direction of propagation. Two special classes of shear waves are defined herein. Specifically, horizontal shear (Sh) waves where the particle motion in the transverse plane is further restricted to be perpendicular to the line of profile of the seismic survey E (i.e., horizontal) and vertical shear (sv) waves where the particle motion in the transverse plane is further restricted to be perpendicular to the horizontal shear (Sh) particle motion all of which is shown in FIG. 1.
Exemplary of the dependence of the orientation of the seismic survey line of profile with respect to the geological character of the subterranean formation, when matched shear wave seismic sources and shear wave seismic receivers are used, it is known by those skilled in the art that shear wave seismic surveys are adversely affected by azimuthally anisotropic subterranean formations. Azimuthally anisotropic subterranean formations have generally vertical parallel planes of symmetry. Because (Sh) waves and (Sv) waves interact differently with the symmetry planes of the azimuthally anisotropic subterranean formation, especially when the symmetry planes are either parallel to or perpendicular to the line of action of the shear wave, care must be taken to ensure that the seismic survey line of profile is laid out either parallel or perpendicular to the symmetry planes.
When the seismic survey line of profile is laid out either parallel or perpendicular to the symmetry planes, the utilization of matched sets of (Sh) wave and (Sv) wave seismic receivers and seismic sources have been used to demonstrate the anisotropic geological character of a subterranean formation. Such a technique requires prior knowledge of the seismic velocity anisotropy of the subterranean formation to be successful.
The interaction differences of (Sh) waves and (Sv) waves have been utilized to detect and measure the anisotropic properties of an azimuthally anisotropic subterranean formation when the seismic lines of profile are properly oriented with respect to the surfaces of the symmetry planes and matched sets of shear wave seismic sources and shear wave seismic receivers have been deployed in the seismic survey. In such applications, (Sh) and (Sv) shear wave seismic sources and seismic receivers are utilized, but only in matched sets, i.e., (Sh) shear wave seismic sources with (Sh) shear wave seismic receivers and (Sv) shear wave seismic sources with (Sv) shear wave seismic receivers.
However, if the seismic survey line of profile is not properly oriented with respect to the planes of symmetry, the seismic information observed can be difficult to interpret at best.
As shown in FIG. 1, the (Sh) wave and (Sv) wave lines of action for the seismic source S and seismic receiver R are defined with respect to the line of profile of the seismic survey E. As such, the orientation of the seismic survey line of profile with respect to the symmetry planes is critical. Consequently, utilization of matched sets of shear wave seismic sources and shear wave seismic receivers have produced inconsistent results when the seismic survey line of profile has not been properly laid out with respect to the anisotropic geological character of the subterranean formations.
Those acquainted with the art of seismic exploration, especially in seismically virgin territory, realize that prior knowledge of the geological character of the subterranean formations is generally not available prior to seismic exploration. The method and system of geophysical exploration of the present invention can be advantageously employed without regard to or knowledge of the geological character of the subterranean formations and still obtain meaningful seismic data.